The present invention concerns metal-glass bonding, particularly when these two materials possess different coefficients of expansion.
In electromagnetic radiation devices, notably X-ray tubes, such bonds are used to permit the passage of electric conductors through the glass walls of an X-ray tube, in order to connect the cathode and anode of the X-ray tube to an electric power source. In fact, the cathode and anode are placed inside a glass wall and are kept under vacuum. The junction between the electric conductors and the glass panel must therefore be tight.
In a known manner, it is possible to use a glass tube, for example, one made of glass manufactured by the SCHOTT company under Nos. 8245 and 8250, sealed with a collar made of Kovar (iron-nickel-cobalt alloy), the thermal expansion coefficients of which are similar. These materials are therefore suited to use in an X-ray tube whose temperatures varies widely, from room temperature to approximately 450.degree. C. However, the cost of such a glass tube is very high.
A Pyrex.RTM. tube and a Kovar collar can also be used. Pyrex.RTM. and Kovar possess different coefficients of expansion, a progressive junction is produced by means of a succession of types of glass having coefficients of expansion which vary from that of Kovar to that of Pyrex.RTM., such as SCHOTT's 8447, 8448, 8449 and 7740 types of glass.
The Pyrex.RTM. tube is therefore equipped at its end bonded to the Kovar collar with a plurality of rings of these different types of glass. It is easily understandable why manufacture of such a Pyrex.RTM. tube is very expensive.